The present invention relates to synchronous motors, and in particular to an improved structure for a synchronous motor which maximizes the available output torque for a motor of given physical size.
Conventionally, fractional horsepower synchronous motors are formed of a pair of tubular or annular electrical coils coaxial with each other, and a cylindrical rotor extended through the center holes of the coils and having an outer peripheral surface of magnetic material defining alternating north and south poles therearound. A pair of plates of magnetically permeable material are positioned adjacent opposite sides of each coil, and have integral flanges formed around the center holes thereof. The flanges of each pair extend toward each other between the inner periphery of the coils and the outer periphery of the rotor, and each pair of flanges defines circumferentially around the rotor a plurality of poles. Upon the application of an a.c. voltage across the coils, adjacent poles formed by the flanges of each pair become of opposite magnetic polarity, and alternate polarity with each half cycle of the a.c. voltage to turn the rotor.
For a fixed operating voltage, the torque which the rotor develops for a given wire size is generally controlled by (a) the number of windings in the coils, which determines the strength of the magnetic field generated at the poles of the flanges associated therewith, and therefore the force exerted by the field on the magnetic material of the rotor, and (b) the diameter of the rotor, which determines the distance from the shaft about which the force acts. In other words, the moment of force about the shaft of the rotor is generally determined by the number of windings of the coils and by the diameter of the rotor.
To increase the torque or moment of force of the rotor, the number of coil windings may be increased, or the diameter of the rotor may be increased, or both. In either event, the net result with a conventionally designed synchronous motor is an increase in the physical size of the motor, either as a result of an increase in the size of the coils as the number of windings is increased, or as a result of an increase in the size of both the rotor and the coils as the diameter of the rotor is increased. In certain uses of synchronous motors, where the physical size of the motor is not critical, an increase in size to achieve greater output torque may be acceptable. In other cases, however, where the area which may be occupied by the motor is limited, an increase in size of the motor may be intolerable.
If, without increasing the physical size of the motor, the diameter of the rotor is increased to increase the distance from the shaft about which the magnetic force acts, then the size of the coils must be reduced to accommodate the larger rotor. Reducing the size of the coils requires decreasing the number of coil windings, which in turn decreases the strength of the magnetic field at their associated poles, and therefore the force that acts on the rotor. Accordingly, although the distance through which the force acts is increased, the force is decreased and thus there is little if any change in torque output.
In the alternative, within given physical dimensions, if the size of the coils is increased by increasing the number of coil windings to generate a magnetic force of greater strength to act on the rotor, the diameter of the rotor must be decreased to accommodate the larger coils. Then, although the magnetic force exerted on the rotor is increased, the distance from the shaft about which the force acts is decreased and again there is little if any change in torque output.